1. Field of the Invention
This invention relates generally to a circuit and apparatus for switching an alternating current (AC) relay at predetermined voltage levels with assured repeatability and precision.
2. The Prior Art
Electromechanical relays with solenoid coils are characterized by a pickup voltage, the voltage at which the solenoid pulls in and energizes the contacts, and a dropout voltage at which the solenoid drops out and de-energizes the contacts. The pickup and dropout voltage for alternating current (AC) relays varies significantly from relay to relay when made by the same manufacturer with even greater differences for relays made by different manufacturers. These differences are due to limitations of control over materials, the size of the relays and assembly techniques.
These limitations are minimal and electrically controllable for direct current (DC) relays. Direct current relays, however, are limited in general applications and cannot be substituted for alternating current relay applications. This invention exploits the controllability and precision of direct current relays and amplifies and transfers the qualities to alternating current relays.
There are several patents that disclose various apparatus and methods for controlling relays.
Graff et al. U.S. Pat. No. 5,267,120 discloses a device whereby a micro-controller measures and then adjusts a time delay in order for the contacts to make at a predetermined point on a sinusoidal waveform.
Hancock, U.S. Pat. No. 4,389,691 discloses an arc suppression device for protecting contacts against excessive wear due to arcing.
Doneghue, U.S. Pat. No. 5,905,422 discloses mechanical means for adjusting the travel distance of the armature of an electromechanical relay to meet the response time parameters.
Brodetsky, U.S. Pat. No. 5,528,120 discloses a field adjustable electronic potential relay for a starting circuit for single-phase motors.
Moan, U.S. Pat. No. 5,633,540 discloses a surge resistant relay switching circuit where the electromechanical relay contacts are protected from inrush currents by placing a TRIAC in series with the switching contacts which blocks current flow until after the contacts are closed.
Lillemo et al, U.S. Pat. No. 5,283,706 discloses a switching circuit whereby a solid state switch forces contacts of an electromechanical relay to open or close at the moment the sine wave crosses at zero in order to prevent arcing and overheating of the contacts.
It therefore is an object of the present invention to provide a device for energizing or de-energizing any alternating current relay at precise pre-determined voltage levels.
It is another object of this invention to provide a device for energizing or de-energizing a solid state switching device capable of switching a current and/or voltage load larger than the current and/or voltage capabilities of the direct current relay contacts.
The present invention is a voltage conditioner and switching device applied to alternating current relays. The present invention is distinguished over the prior art in general and these patents in particular by a simple rectifier circuit which by fixed resistor selection and capacitor filtering determine the precise energizing voltage of a direct current relay. The direct current relay contacts can then energize or de-energize an alternating current relay at consistently the same voltage level regardless of manufacturing material or assembly technique differences. A solid state switching device such as a MOSFET, TRIAC, SCR or other device may be triggered by the direct current relay if the alternating current relay solenoid amperage and/or voltage requirements exceed the direct current relay contact ratings.